Wednesday, January 26, 2011

Chemistry class today was extremely jam-packed and educational today. Mr. H began class by requesting us to take out our calculadoras and paquetos which is all we would need for the day. He told us we would begin the packet with page 13 and explained that the 2 problems we would do on page 14 directly pertained to the Webassign due Friday morning. If you did that Webassign by Thursday morning you would receive extra credit points. Next we went over Alex's blog and Mr. H briefly covered the main ideas of the past few days. We then went straight to the bottom of page 13 in the packet. Mr. H explained the calculations you would need to solve a quantity of heat problem when a substance changes temperature (from one state of being to another). For Q1, Q3, and Q5 which show positive change in both time and temperature, you use the basic q=m*c*ΔT equation. The mass is the mass of the substance that is changing, the c is the specific heat of the substance at that state, and the delta T is the change in temperature that the substance is experiencing as it changes state. Mr. H then clued us in the fact that the specific heat of water at the solid state is 2.05 J/g°C, at the liquid state is 4.18 J/g°C and at the gaseous state is 1.88 J/g°C. For Q2 and Q4 you use an enthalpy equation of either fusion or vaporization. The equation for fusion is q=nΔHfusion and the equation for vaporization is nΔHvaporization. To find the q values of Q2 and Q4 you convert the grams of the given substance to moles using the molar mass and then to kJ's using the heat of fusion or heat of vaporization values that are given to you. MAKE SURE YOU THEN CONVERT ALL THE JOULES VALUES TO EITHER kJ's OR J's. After finding each value you add up each quantity of heat value per step to find the quantity of heat for the entire change. Next we moved on to page 14 to do problems 29 and 30. Given the info, we set up three equations to find Q1, Q2, and Q3. The value of Q1 was 1045 J's which we converted to 1.04 kJ's. The value of Q2 was 10.9 kJ's and the value of Q3 was 4524 J's which we converted to 4.5 kJ's. Then adding each quantity together we got 16.5 kJ's of heat. Number 30 was directly related to the previous equation. We need to find the quantity of heat for a Q1, Q2, Q3, Q4, and Q5 change. However we already knew the values for Q1(1.04 kJ's) and Q2 (10.9 kJ's). We then solved for the new Q3 as well as Q4 and Q5. The new value of Q3 was 13.8 kJ's, the value of Q4 was 74.39 kJ's and the value of Q5 was 1.5 kJ's. Adding all 5 values together we got 102 kJ's of heat. While we were calculating number 30 Mr. H did a demo type experiment where he would rub water on the table and it would evaporate. He also did this with Acetone which is similar to nail polish remover and it evaporated in seconds. Mr. H explained that liquids turn to gases naturally even when they aren't heated and are able to "jump off" the table although it was obviously not 100 degrees Celsius in the room. Next Mr. H went over the vapor pressure of water curve. He told us that page 233 in the textbook has the best picture of it. Basically if you want to know the pressure at a specific degree you just follow the graph up and across. The graph stresses the point that depending what the pressure is, the point or temperature at which a substance changes to another state is different. Next we went to page 32 in the packet and Mr. H listed the things we should be able to note on a phase diagram. He also wrote that normal bp temperature at which the vapor pressure of a liquid=atmospheric pressure of 1 atm. Mr. H finished class with a demo. He had previously been boiling water in a beaker and then removed it from the heat and stoppered. He flipped it upside down on a rack and after adding ice cubes to the top of beaker (in contact with the gas not the liquid) the water began to boil again. It was apparent that the water was not at 100 degrees Celsius so the reason it boiled was because although the temperature went down the pressure did as well so we were able to change the boiling point. (see figure 9.5 in textbook). Mr. H then briefly mentioned that the reason people could walk on coals or other hot things was because they put water on their feet beforehand so the water absorbs the heat and their skin doesn't. Right before class ended we went over the top of page 15 about phase diagrams. First we labeled the different sections as either solid, liquid, or gas and then using the graph we found the answers to 2 which were vapor, solid, solid, and liquid. For homework we had a reading Webassign and a Webassign due Friday worth extra credit. There is a pop quiz on Friday.